The disclosure concerns a method to operate a digital printer to print a recording material with toner particles that are applied with the aid of a liquid developer, in particular a high-speed printer to print web-shaped or sheet-shaped recording media. Furthermore, the disclosure concerns a digital printer to determine the toner concentration.
In such digital printers, a latent charge image of a charge image carrier is inked by means of electrophoresis with the aid of a liquid developer. The toner image that is created in such a manner is transferred indirectly (via a transfer element) or directly to the recording material. The liquid developer has toner particles and carrier fluid in a desired ratio. Mineral oil is advantageously used as a carrier fluid. In order to provide the toner particles with an electrostatic charge, charge control substances are added to the liquid developer. Further additives are additionally added, for example in order to achieve the desired viscosity or a desired drying behavior of the liquid developer.
Such digital printers have been known for a long time, for example from DE 10 2010 015 985 A1, DE 10 2008 048 256 A1 or DE 10 2009 060 334 A1.
From the document DE 10 2008 047 196 A1 by the same applicant, a method is known to determine the concentration of toner particles for a liquid developer system in which the suspension of carrier fluid and toner particles is charged with ultrasonic pulses. To determine the toner concentration, the travel time of the ultrasonic pulses is measured in a measurement cell, wherein a delay time period is initially determined from the digital clock signal of a microcomputer. After the delay time has elapsed, a capacitor is charged with a voltage until a first zero crossing of the signal received by the ultrasonic receiver takes place. The travel time of the ultrasonic pulses is determined with high resolution from the sum of delay period and charging time, and the toner concentration is concluded from this. The method described in this document has proven itself in practice; however, the determination of the toner concentration is strongly dependent on the temperature of the suspension. Moreover, the acoustic attenuation (which changes given a change of the toner concentration) affects the receiver signal, and the detection of the zero crossing of the attenuated signal for precise travel time determination is difficult.
The document U.S. Pat. No. 7,570,893 B2 describes a method and a device to monitor a developer fluid in order to therefore determine the temperature and the toner concentration of the developer fluid. The sound velocity and the attenuation of the developer fluid are measured with the aid of an ultrasound sensor, wherein both variables are dependent to different degrees on the temperature, such that the temperature can be calculated from both measurement variables. The attenuation of the ultrasonic wave is calculated from the ratio of the amplitudes of a first freely propagating ultrasonic wave and a second, reflected ultrasonic wave.